When The Boston Globe launches health, medicine and life science news site Stat this fall, it will be the third such niche news site for the Globe. Those news sites are all part of the Globe’s strategy to attract more national and international readers, Ricardo Bilton writes. On how the Globe decides which verticals to pursue, Boston Globe Media Partners CEO Mike Sheehan says: “These are the things for which Boston is known for and what we think we can own. When we go into verticals in which we’re were dominant, it expands our overall audience, which helps boost everything else. You’re never going to see us start a site focused on consumer packaged goods.”